Method of and means for manufacturing sulphite cellulose



June 23, 1959 K. KLINGA 2,891,886

METHOD OF AND MEANS FOR MANUFACTURING SULPHITE CELLULOSE Filedflune 4, 1956 33W w or Ira/er NETHOD OF AND MEANS FOR MANUFACTUR- ING SULPHITE CELLULCSE Karl Ivar Klinga, Uppsala, Sweden, assignor to Aktiebolaget Celleco, Uppsala, Sweden Application June 4, 1956, Serial No. 589,044 Claims priority, application Sweden June 13, 1955 4 Claims. (Cl. 16237) The present invention relates to improvements in the manufacture of sulphite cellulose.

It is usual practice in the cooking of sulphite cellulose to add with the cooking liquor a surplus of sulphur dioxide above what may be chemically combined with the spent liquor in the course of the cooking operation. Said surplus is recovered for economical and sanitary reasons during and subsequent to the cooking operation proper to be used for preparing another batch of cooking liquor. Since the sulphur dioxide is in a gaseous state under normal pressure and temperature conditions, it will be absorbed during and subsequent to the cooking operation by cooking liquor under preparation, thereby increasing its content of sulphur dioxide to the value desired for the cooking liquor. Thus, in order to obtain a frictionless operation it is necessary to recover sulphur dioxide at the same rate as newly prepared cooking liquor is required for the cooking process. This, of course, being not always the case, there are large tanks provided for accumulating ready prepared cooking liquor as well as cooking liquor in various stages of preparation, in order to allow equalization of possible unbalances between supply and consumption of sulphur dioxide recovered from the cooking operation.

It has long been desired to collect the recovered sulphite dioxide as such in some kind of accumulator which with moderate dimensions combines the equalization quality of the large cooking liquor containers. Since the sulphur dioxide, as a rule, is in a gaseous state, the accumulator might take the form of a gasometer. Such a device, however, would be both expensive and troublesome owing to the extremely corrosive qualities of the sulphur dioxide. To compress the sulphur dioxide in the usual way into liquid state by means of a compressor is also conceivable but hardly practicable, substantially because of the fact that there is probably no lubricating oil available which may withstand the influence of moisty sulphur dioxide during any longer time without disabling the compressor by the formation of pitch therein. It has further been proposed in a system for carrying out a fractional condensation of gases and steam escaping from the digester to first remove by condensation at a suitable temperature substantially the whole amount of steam and then to still reduce the temperature of the remaining gas mixture and recover the sulphur dioxide in liquid state by condensation, the pressure as required to perform said condensation being obtained from the digester under pressure, that is to say, the pressure above atmospheric existing in the condensation system is equal to that existing in the digester.

The last mentioned method, however, may only etiect a partial condensation of the sulphur dioxide owing to the presence in the gas mixture of carbon dioxide and residues, especially nitrogen, produced during the cooking operation by air contained in the wooden chips. In many cases, especially in summer, cooling water of suitable temperature for the condensation of S that is, at most l0-12 C. at the hitherto most common,

2,891,886 Patented June 23, 1959 2 allowable ressure of 5 lip/cm. in the sulphlte dlgesters,

is not available. To overcome this drawback a method has been proposed, according to which the sulphur dioxide contained in the digester exhaust gases is absorbed, after cooling, into water under the same pressure as that existing in the digester. Such a method is more independent of the temperature of the water and may yield, very strong watery solutions of sulphur dioxide, at the same time allowing removal from the recovery system of the troublesome incondensable gases, viz. carbon di oxide and air residues (nitrogen). The volume of the sulphur dioxide solution will yet be at least 4-5 times larger than the volume that the included sulphur dioxide, after condensation, to liquid state would occupy. As a further drawback, the water would constitute an undesired dead-weight also from other points of view.

The present invention has for its object to avoid the above stated drawbacks and to deliver the whole sulphur dioxide content of the digester exhaust gases in a prac tically liquid state. The invention has reference, specifically, to a sulphite cellulose manufacturing system in which at least 200 lbs. free S0 per short ton pulp, oven dry, are admitted to the digester with the cooking liquor or in another way and in which the gases delivered from the digester during the cooking operation are utilized in preparing cooking liquid. The method is illustrated by the diagram shown in the accompanying drawing. The sulphite digester 1 delivers continuously during the period of highest pressure of the cooking operation sulphur dioxide together with steam and diluting gases, substantially carbon dioxide and nitrogen. The mixture flows through the trap 2 where possibly withdrawn pulp and liquor are removed. Thence, the mixture passes to the heat exchanger or cooler 3 where it is cooled by water or raw liquor to about 40 C. Since the gas mixture is still under practically the same pressure above atmospheric as that existing within the digester, usually about 5 kg./cm. the steam is here condensed to 98-99%, so that the remaining gas will be almost dry. Condensate and gases are separated in well known way in a separator 4 due to the fact that the rate of flow of the gas is reduced owing to the larger diameter of the separator, so as to allow small condensate drops that might be floating in the gas to separate and flow ofi together with the other condensate, that is to say, water, saturated, of course, with dissolved sulphur dioxide, to the receptacle 9.

Gases which have not'been condensated in the cooler 3 pass to the condenser 5. Here, they are cooled by means of cold water, causing sulphur dioxide to condensate in a proportion that depends on the pressure, the temperature and the content of other gases. In the separato-r 6 which may be of the same design as the separator 4, gases and liquid matters are separated from each other. The liquid matters consisting of S0 in liquid state, flow to the receptacle 7 and the gases, which still contain up to 50-60 percent by volume of sulphur dioxide, pass to an absorber 8, which may be of usual design, comprising, for instance, a column packed with ceramic filling bodies sprayed with water alone from the top of the column or a column having a set of bell-shaped bottoms in which the absorbing water is admitted onto the uppermost bottom. The sulphur dioxide content of the gases is absorbed by the water during its passage downwardly through the column, while gases practically free from sulphur dioxide are allowed to escape to the atmosphere through a valve 10 at the top of the column. Said valve acts at the same time to control the pressure of the whole system including the digester or digesters so that the pressure in the system up to valve 10 will be practically equal to that existing in the digester or digesters. As a result, the SO -solution as obtained in the absorption column will be a highly concentrated state. In certain cases a cooling of the sulphur dioxide solution may be required as an intermediate stage of the absorption process, since the heat of dissolution of the sulphur dioxide may raise the temperature of the solution by about 1 C. for each percent of sulphur dioxide absorbed. The solution resulting is passed to the receptacle 9.

In order to recover the sulphur dioxide from the watery solution contained in the receptacle 9 said solution is passed to an evaporator 11 of a structure similar to that of the absorption column 8 and, if desired, provided with a return flow cooler 12. The solution is introduced at the top of the evaporator below the return flow cooler. At the bottom steam is supplied at such a pressure, at least, as to allow the gaseous sulphur dioxide evaporated in 11 to enter the separator 4 and there combine with the gas rich in sulphur dioxide which is passed to the condenser 5 in order to be there transformed into liquid state and then collected in the receptacle 7. in this way the whole amount of sulphur dioxide as delivered from the digester 1 is recovered in a practically up to 100% liquid state. From the bottom of the evaporator ill hot water free from S0 is discharged.

As indicated by arrows in the diagram the liquid sulphur dioxide collected in the receptacle 7 is returned to the digester in order to enrich the content of sulphur dioxide of the cooking liquor. Also the water solution of sulphur dioxide collected in the receptacle 9 may be returned in the same way, if desired. The evaporation in 11 may thus be dispensed with. it is evident that the accumulation of the sulphur dioxide in a practically up to 100% liquid state is an extraordinary means for controlling the composition of the cooking liquid as well as for accumulating considerable quantities of sulphur dioxide within a moderate volume, to allow satisfaction of unforseen Wants outside the cooking plant, as for instance, in a bleaching plant for the pulp. The liquid sulphur dioxide is also a necessary expedient in the carrying out of certain methods of cooking sulphite pulp in two stages recently put into practice. It is also to be noted that a recovery of cymol produced during the cooking operation would, no doubt, be effected to ad vantage by skimming if off from the sulphur dioxide solution collected in the receptacle 9.

It is evident to those skilled in the art that the method above described may be employed also in the hydrolysis of carbohydrates-containing material, such as wood, to sugar with sulphur dioxide under pressure in connection with the recovery of sulphur dioxide.

I claim:

1. In a sulphite cellulose manufacturing system, in which at least 200 lbs. free S0 per short ton pulp, oven dry, are admitted to the digester, and in which the gases delivered from the digester during the cooking operation are utilized in preparing cooking liquor, the method comprising subjecting said digester gases during the period of the cooking operation, during which the digester is subjected to its highest pressure, to an indirect cooling operation to lower their temperature to such a value of about 4945 C., as to cause condensation only of the steam included in the gases and delivering it in liquid state, passing the nearly dry gases resulting after said cooling operation to a condenser, further cooling them therein to about 1520 C. for effecting a partial condensation and separation of the sulphur dioxide included in the gases, passing the separated sulphur dioxide to an accumulator, causing absorption in water alone of the sulphur dioxide content of the gas mixture under substantially the same pressure as that existing in the digester, carrying the resulting gases free from sulphur dioxide off from the top of the absorption apparatus and allowing it to escape to the atmosphere Via an adjustable valve serving to control the pressure inside the digester, and passing a highly concentrated watery solution of sulphur dioxide from the bottom of the absorption apparatus to an accumulator holding substantially the same pressure as the digester.

2. A method as claimed in claim 1, in which the concentrated watery solution of sulphur dioxide as obtained from the accumulator is freed from its content of sulphur dioxide by distilling it under such a pressure as to allow the gaseous sulphur dioxide leaving the distilling apparatus to be directly restored to the gases leaving the digester during the cooking operation to be then condcnsated to liquid state.

3. A system for manufacturing sulphite cellulose comprising the combination with a digester and a pressure control valve therefor, of a cooler, a first separator, a condenser, a second separator and an absorbing column inserted in the stated order between the digester and the control valve and in such a way as to allow the digester gas to pass in counter current to the absorbing liquid through said column, while the control valve maintains in the cooler, the first separator, the condenser, the second separator and the column a pressure of substantially the same order as the pressure existing inside the digester.

4. A system as claimed in claim 3 which includes after the said column an accumulator for highly concentrated watery solution of sulphur dioxide, characterized by the provision between said accumulator and the first separator of a distillating apparatus in which the highly concentrated sulphur dioxide solution passes in counter current to steam for removing the sulphur dioxide by distillation.

References Cited in the file of this patent UNITED STATES PATENTS 1,119,977 Moore Dec. 8, 1914 1,159,352 Bergstrom Nov. 9, 1915 1,213,414 Richter Jan. 23, 1917 1,852,011 Hatch Apr. 5, 1932 2,235,759 Frankel Mar. 18, 1941 2,802,791 Whitney Aug. 13, 1957 

1. IN A SULPHITE CELLULOSE MANUFACTURING SYSTEM, IN WHICH AT LEAST 200 LBS. FREE SO2 PER SHORT TON PULP, OVEN DRY, ARE ADMITTED TO THE DIGESTER, AND IN WHICH THE GASES DELIVERED FROM THE DIGESTER DURING THE COOKING OPERATION ARE UTILIZED IN PREPARING COOKING LIQUOR, THE METHOD COMPRISING SUBJECTING SAID DIGESTER GASES DURING THE PERIOD OF THE COOKING OPERATION, DURING WHICH THE DIGESTER IS SUBJECTED TO ITS HIGEST PRESSURE, TO AN INDIRECT COOLING OPERATION TO LOWER THEIR TEMPERATURE TO SUCH A VALUE OF ABOUT 40-45*C., AS TO CAUSE CONDENSATION ONLY OF THE STEAM INCLUDED IN THE GASES AND DELIVERING IT IN LIQUID STATE, PASSING THE NEARLY DRY GASES RESULTING AFTER SAID COOLING OPERATION TO A CONDENSER, FURTHER COOLING THEM THEREIN ABOUT 15*-20*C. FOR EFFECTING A PARTIAL CONDENSATION AND SEPARATION OF THE SULPHUR DIOXIDE INCLUDED IN THE GASES, PASSING THE SEPARATED SULPHUR DIOXIDE TO AN ACCUMULATOR, CAUSING ABSORPTION IN WATER ALONE OF THE SULPHUR DIOXIDE CONTENT OF THE GAS MIXTURE UNDER SUBSTANTIALLY THE SAME PRESSURE AS THAT EXISTING IN THE DIGESTER, CARRYING THE RESULTING GASES FREE FROM SULPHUR DIOXIDE OFF FROM THE TOP OF THE ABSORPTION APPARATUS AND ALLOWING IT TO ESCAPE TO THE ATMOSPHERE VIA AN ADJUSTABLE VALVE SERVING TO CONTROL THE PRESSURE INSIDE THE DIGESTER, AND PASSING A HIGHLY CONCENTRATED WATERY SOLUTION OF SULPHUR DIOXIDE FROM THE BOTTOM OF THE ABSORPTION APPARATUS TO AN ACCUMULATOR HOLDING SUBSTANTIALLY THE SAME PRESSURE AS THE DIGESTER. 